The invention discloses an oil-immersed type transformer solid insulation moisture content assessment method based on return voltage, comprising the following steps: 1) preparation work: the temperature of a transformer is lowered without forced oil circulation; an outgoing line terminal and a line of each winding sleeve are separated; and each phase of a high voltage winding and neutral points are in short circuit and grounded; 2) wire connection: each measured winding is in grounded insulation; 3) parameter configuration; 4) test end judgment: a polarized spectrum curve raises smoothly and then declines, when the peak value of the polarized spectrum curve appears, test is ended; when the tested polarized spectrum curve has serious jump or a plurality of peak values, the test is ended when the charging time Tc equals to 100s; 5) moisture content is judged according to a main time constant of the polarized spectrum curve; and 6) when the moisture content is less than 1%, the insulation state is good; when the moisture content ranges from 1% to 2.5%, the insulation state is general; and when the moisture content is higher than 2.5%, the insulation state is very poor. The invention can accurately assess brackish moisture content in insulation paper and judge the insulation and aging state of oil paper without suspended core and sampling and destroying the insulation material.